Conventional power transfer mainly depends on a form of wire connection, but wires occupy large amounts of space resources, consume large quantities of metal resources, meanwhile easily produce contacting electric spark, and cause great hidden safety problems to living and life. In addition, in some utilization occasions, the wires could not be utilized for power transfer, but the using of batteries greatly restricts the operating life of devices. Under such backgrounds, people start trying a new power transfer technology, that is, wireless power transfer technology.
In 2007, scientific research personnel at Massachusetts Institute of Technology firstly utilized magnetic resonant coupling to entirely lighten a lamp bulb of 60 W in the case of a distance of 2 m. This technology is named WiTricity technology, and opens a new direction for the development of wireless power transfer technology within a medium range distance. However, the disadvantage of this system is that a fixed distance is needed, and when the distance changes, the system efficiency will be lowered greatly. A great many of succeeding researches all have made improvements based on this. In August 2008, Joshua R. Smith's research group at Intel Seattle laboratory designed a planar-type wireless power transfer device for charging small electronic devices by utilizing wireless power transfer technology with magnetic resonant coupling, showed a system realizing lightening a lamp bulb of 40W at a distance of 60 cm, and published a high-level SCI paper in Industrial Electronics Society magazine in 2011. The system is a system with a variable distance based on frequency-tracking, which could realize a high-efficiency transmission over 75% within a distance of 75 cm. But circuits such as frequency measurement circuit, phase-locked loop circuit and voltage sampling control circuit etc. are needed to be added to the system based on the original WiTricity system, which increases the complexity of the system, and causes some difficulties for realizing debugging and application. In 2014, in a literature <Lee W, Oh K, Yu J, “Distance-Insensitive Wireless Power Transfer and Near-Field Communication Using a Current-Controlled Loop With a Loaded Capacitance,” IEEE Transactions on Antennas & Propagation, 2014, 62(2):936-940.>, an antiparallel resonant loop was put forward, which realized an efficiency over 60% within a distance between 0 to 7 cm. Nevertheless, the antiparallel loops weaken the mutual inductances in the weak coupling region and, consequently, of the extension of the operating distance.